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                <h1 id="1-知识点回顾"><a href="#1-知识点回顾" class="headerlink" title="1. 知识点回顾"></a>1. 知识点回顾</h1><p>队列（queue）是一种只能在一端插入元素、在另一端删除元素的数据结构，遵循先入先出（FIFO）的规则。</p>
<p>环形队列（ring queue）可以方便的重复利用这段内存空间，同样遵循先入先出（FIFO）的规则。</p>
<p>优先级队列（prio queue）不遵循FIFO，而是根据元素的优先级进行出队，优先级最高的先出队。</p>
<p><strong>本文的所有内容都是基于这两个数据结构</strong>，TencentOS-tiny中环形队列和优先级队列的实现和使用示例请阅读文章：</p>
<ul>
<li><a target="_blank" rel="noopener" href="https://mculover666.blog.csdn.net/article/details/107427737">数据结构 | TencentOS-tiny中队列、环形队列、优先级队列的实现及使用</a></li>
</ul>
<h1 id="2-消息队列"><a href="#2-消息队列" class="headerlink" title="2. 消息队列"></a>2. 消息队列</h1><h2 id="2-1-什么是消息队列"><a href="#2-1-什么是消息队列" class="headerlink" title="2.1. 什么是消息队列"></a>2.1. 什么是消息队列</h2><p>消息队列，Message Queue，顾名思义包含两部分：消息+队列，或者可以理解为消息的队列。</p>
<p>① 消息是什么？</p>
<p>两个不同的任务之间传递数据时，这个数据就称之为消息，这个消息可以是一个整型值，浮点值，甚至一个结构体，一个指针……所以，在使用不同的RTOS的消息队列时，<strong>一定要注意传递的是值还是该值的地址</strong>。</p>
<p>传递值的缺点是值的长度有大有小，导致整个消息队列的长度有大有小。</p>
<p>一个指针的长度是固定的4字节，传递值的时候，无论值是什么类型，只传递该值的地址。</p>
<p>传递地址当然也有缺陷，当动态任务task1中定义了一个局部变量，然后把该局部变量的地址传给了task2，随即task1因为某种原因被销毁，内存回收，导致指向该局部变量的指针变为野指针，非常危险，不过不用慌，小问题，在编程的时候注意避免即可。</p>
<p><strong>在TencentOS-tiny中，消息队列中传递的消息指的是地址，邮箱队列传递的消息是值</strong>。</p>
<p>② 队列是什么？</p>
<p>消息队列如果底层使用环形队列存储消息，则成为消息队列，遵循：先送入的消息先被取出。</p>
<p>消息队列如果底层使用优先级队列存储消息，则成为优先级消息队列，遵循：优先级最高的消息最先被取出。</p>
<p><strong>在TencentOS-tiny中，这两种消息队列都有，下面一一讲述。</strong></p>
<p>③ pend-post机制</p>
<p>无论是什么队列，都存在两种情况：当队列满了的时候，元素再入队会发生错误；当队列为空的时候，元素出队同样会发生错误。</p>
<p>这种问题可以巧妙的在队列基础之上用pend-post机制解决，即等待-释放机制。</p>
<p>当队列<strong>满了</strong>的时候，前来入队的task1可以选择pend一段时间或者永久等待，<strong>一旦有元素被task2出队</strong>，调用post释放一个信号，<strong>唤醒等待中的task1</strong>。</p>
<p>同样，当队列<strong>空了</strong>的时候，前来出队的task1可以选择pend一段时间或者永久等待，<strong>一旦有元素被task2入队</strong>，调用post释放一个信号，<strong>唤醒等待中的task1</strong>。</p>
<p>是不是很巧妙？</p>
<p>接下来上源码！上Demo！一看便知~</p>
<h2 id="2-2-消息队列的实现"><a href="#2-2-消息队列的实现" class="headerlink" title="2.2. 消息队列的实现"></a>2.2. 消息队列的实现</h2><p>TencentOS-tiny中消息队列的实现在 <code>tos_message_queue.h</code>和<code>tos_message_queue.c</code>中。</p>
<pre><code class="c">typedef struct k_message_queue_st &#123;
    knl_obj_t   knl_obj;

    pend_obj_t  pend_obj;
    k_ring_q_t  ring_q;
&#125; k_msg_q_t;
</code></pre>
<p>一个pend_obj对象用来实现pend-post机制，一个ring_q环形队列用来存储消息。</p>
<p>是不是和我讲述的没错？学透了之后，其实一切都没有那么神秘的~</p>
<p>再来看看从消息队列中获取消息的API实现：</p>
<pre><code class="c">__API__ k_err_t tos_msg_q_pend(k_msg_q_t *msg_q, void **msg_ptr, k_tick_t timeout)
&#123;
    //省略了部分源码
    TOS_CPU_INT_DISABLE();

    if (tos_ring_q_dequeue(&amp;msg_q-&gt;ring_q, msg_ptr, K_NULL) == K_ERR_NONE) &#123;
        TOS_CPU_INT_ENABLE();
        return K_ERR_NONE;
    &#125;
    
    pend_task_block(k_curr_task, &amp;msg_q-&gt;pend_obj, timeout);

    TOS_CPU_INT_ENABLE();
    knl_sched();

    return err;
&#125;
</code></pre>
<p>向消息队列中存放消息的API实现如下：</p>
<pre><code class="c">__STATIC__ k_err_t msg_q_do_post(k_msg_q_t *msg_q, void *msg_ptr, opt_post_t opt)
&#123;
    //省略了部分源码
    TOS_CPU_INT_DISABLE();

    if (pend_is_nopending(&amp;msg_q-&gt;pend_obj)) &#123;
        err = tos_ring_q_enqueue(&amp;msg_q-&gt;ring_q, &amp;msg_ptr, sizeof(void*));
        if (err != K_ERR_NONE) &#123;
            TOS_CPU_INT_ENABLE();
            return err;
        &#125;
        TOS_CPU_INT_ENABLE();
        return K_ERR_NONE;
    &#125;

    if (opt == OPT_POST_ONE) &#123;
        msg_q_task_recv(TOS_LIST_FIRST_ENTRY(&amp;msg_q-&gt;pend_obj.list, k_task_t, pend_list), msg_ptr);
    &#125; else &#123; // OPT_POST_ALL
        TOS_LIST_FOR_EACH_ENTRY_SAFE(task, tmp, k_task_t, pend_list, &amp;msg_q-&gt;pend_obj.list) &#123;
            msg_q_task_recv(task, msg_ptr);
        &#125;
    &#125;

    TOS_CPU_INT_ENABLE();
    knl_sched();

    return K_ERR_NONE;
&#125;
</code></pre>
<p>从源码中可以看到，如果opt标志为 OPT_POST_ONE，表示唤醒一个，则唤醒该消息队列等待列表上任务优先级最高的那个；如果opt标志为 OPT_POST_ALL，则全部唤醒。</p>
<h2 id="2-3-消息队列的使用示例"><a href="#2-3-消息队列的使用示例" class="headerlink" title="2.3. 消息队列的使用示例"></a>2.3. 消息队列的使用示例</h2><pre><code class="c">#define MESSAGE_MAX     10

uint8_t msg_pool[MESSAGE_MAX * sizeof(void *)];

k_msg_q_t msg_q;

void entry_task_receiver(void *arg)
&#123;
    k_err_t err;
    void *msg_received;

    while (K_TRUE) &#123;
        err = tos_msg_q_pend(&amp;msg_q, &amp;msg_received, TOS_TIME_FOREVER);
        if (err == K_ERR_NONE) &#123;
            printf(&quot;receiver: msg incoming[%s]\n&quot;, (char *)msg_received);
        &#125;
    &#125;
&#125;

void entry_task_sender(void *arg)
&#123;
    char *msg_prio_0 = &quot;msg 0 without priority&quot;;
    char *msg_prio_1 = &quot;msg 1 without priority&quot;;
    char *msg_prio_2 = &quot;msg 2 without priority&quot;;

    printf(&quot;sender: post a message 2 without priority\n&quot;);
    tos_msg_q_post(&amp;msg_q, msg_prio_2);

    printf(&quot;sender: post a message 1 without priority\n&quot;);
    tos_msg_q_post(&amp;msg_q, msg_prio_1);

    printf(&quot;sender: post a message 0 without priority\n&quot;);
    tos_msg_q_post(&amp;msg_q, msg_prio_0);
&#125;
</code></pre>
<p>执行结果如下：</p>
<pre><code class="c">TencentOS-tiny Port on STM32L431RCT6 By Mculover666
sender: post a message 2 without priority
sender: post a message 1 without priority
sender: post a message 0 without priority
receiver: msg incoming[msg 2 without priority]
receiver: msg incoming[msg 1 without priority]
receiver: msg incoming[msg 0 without priority]
</code></pre>
<h1 id="3-优先级消息队列"><a href="#3-优先级消息队列" class="headerlink" title="3. 优先级消息队列"></a>3. 优先级消息队列</h1><h2 id="3-1-优先级消息队列的实现"><a href="#3-1-优先级消息队列的实现" class="headerlink" title="3.1. 优先级消息队列的实现"></a>3.1. 优先级消息队列的实现</h2><p>实现和消息队列类似，通过在优先级队列的基础上加上pend-post机制来实现。</p>
<p>TencentOS-tiny中优先级消息队列的实现在<code>tos_priority_message_queue.h</code>和<code>tos_priority_message_queue.c</code>中。</p>
<pre><code class="c">typedef struct k_priority_message_queue_st &#123;
    knl_obj_t   knl_obj;

    pend_obj_t  pend_obj;

    void       *prio_q_mgr_array;
    k_prio_q_t  prio_q;
&#125; k_prio_msg_q_t;
</code></pre>
<p>其中pend_obj用于挂载等待该优先级消息队列的任务，prio_q和prio_q_mgr_array合起来实现优先级队列。</p>
<p>消息入队和消息出队的API实现与消息队列的实现思想一模一样，这里不再讲解。</p>
<h2 id="3-2-优先级消息队列的使用示例"><a href="#3-2-优先级消息队列的使用示例" class="headerlink" title="3.2. 优先级消息队列的使用示例"></a>3.2. 优先级消息队列的使用示例</h2><pre><code class="c">#define MESSAGE_MAX     10

uint8_t msg_pool[MESSAGE_MAX * sizeof(void *)];

k_prio_msg_q_t prio_msg_q;

void entry_task_receiver(void *arg)
&#123;
    k_err_t err;
    void *msg_received;

    while (K_TRUE) &#123;
        err = tos_prio_msg_q_pend(&amp;prio_msg_q, &amp;msg_received, TOS_TIME_FOREVER);
        if (err == K_ERR_NONE) &#123;
            printf(&quot;receiver: msg incoming[%s]\n&quot;, (char *)msg_received);
        &#125;
    &#125;
&#125;

void entry_task_sender(void *arg)
&#123;
    char *msg_prio_0 = &quot;msg with priority 0&quot;;
    char *msg_prio_1 = &quot;msg with priority 1&quot;;
    char *msg_prio_2 = &quot;msg with priority 2&quot;;

    printf(&quot;sender: post a message with priority 2\n&quot;);
    tos_prio_msg_q_post(&amp;prio_msg_q, msg_prio_2, 2);

    printf(&quot;sender: post a message with priority 1\n&quot;);
    tos_prio_msg_q_post(&amp;prio_msg_q, msg_prio_1, 1);

    printf(&quot;sender: post a message with priority 0\n&quot;);
    tos_prio_msg_q_post(&amp;prio_msg_q, msg_prio_0, 0);
&#125;
</code></pre>
<p>运行结果如下：</p>
<pre><code class="c">TencentOS-tiny Port on STM32L431RCT6 By Mculover666
sender: post a message with priority 2
sender: post a message with priority 1
sender: post a message with priority 0
receiver: msg incoming[msg with priority 0]
receiver: msg incoming[msg with priority 1]
receiver: msg incoming[msg with priority 2]
</code></pre>
<blockquote>
<p>将第2节的结果和第3节的结果对比，就会发现同样的消息发送顺序，因为使用不同的消息队列，任务获取到的消息顺序截然不同。</p>
</blockquote>
<h1 id="4-邮箱队列"><a href="#4-邮箱队列" class="headerlink" title="4. 邮箱队列"></a>4. 邮箱队列</h1><h2 id="4-1-不同之处"><a href="#4-1-不同之处" class="headerlink" title="4.1. 不同之处"></a>4.1. 不同之处</h2><p>消息队列和邮箱队列的不同之处，在于底层队列每个元素类型不一样，看一眼源码便知。</p>
<p>消息队列传递的消息是地址，所以在初始化消息队列的时候，环形队列中每个元素都是空指针类型：</p>
<pre><code class="c">__API__ k_err_t tos_msg_q_create(k_msg_q_t *msg_q, void *pool, size_t msg_cnt)
&#123;
    //部分源码省略

    //重点：队列中每个元素类型大小是sizeof(void*)
    err = tos_ring_q_create(&amp;msg_q-&gt;ring_q, pool, msg_cnt, sizeof(void *));
    if (err != K_ERR_NONE) &#123;
        return err;
    &#125;

    return K_ERR_NONE;
&#125;
</code></pre>
<p>而邮箱队列传递的是值，所以在初始化底层用到的环形队列时，每个元素的大小是由用户指定的：</p>
<pre><code class="c">__API__ k_err_t tos_mail_q_create(k_mail_q_t *mail_q, void *pool, size_t mail_cnt, size_t mail_size)
&#123;
    //省略了部分源码
    
    //重点：每个元素的大小是mail_size，由用户传入参数指定
    err = tos_ring_q_create(&amp;mail_q-&gt;ring_q, pool, mail_cnt, mail_size);
    if (err != K_ERR_NONE) &#123;
        return err;
    &#125;

    return K_ERR_NONE;
&#125;
</code></pre>
<h2 id="4-2-邮箱队列的实现"><a href="#4-2-邮箱队列的实现" class="headerlink" title="4.2. 邮箱队列的实现"></a>4.2. 邮箱队列的实现</h2><p>这有什么好实现的~一个环形队列+pend-post对象即可。</p>
<p>TencentOS-tiny中邮箱队列的实现在<code>tos_mail_queue.h</code>和<code>tos_mail_queue.c</code>中。</p>
<pre><code class="c">typedef struct k_mail_queue_st &#123;
    knl_obj_t   knl_obj;

    pend_obj_t  pend_obj;
    k_ring_q_t  ring_q;
&#125; k_mail_q_t;
</code></pre>
<p>是不是没什么区别~至于操作的API，更没啥区别，不写了，划水划水。</p>
<h2 id="4-3-邮箱队列的使用示例"><a href="#4-3-邮箱队列的使用示例" class="headerlink" title="4.3. 邮箱队列的使用示例"></a>4.3. 邮箱队列的使用示例</h2><pre><code class="c">#define MAIL_MAX    10

typedef struct mail_st &#123;
    char   *message;
    int     payload;
&#125; mail_t;

uint8_t mail_pool[MAIL_MAX * sizeof(mail_t)];

k_mail_q_t mail_q;

void entry_task_receiver_higher_prio(void *arg)
&#123;
    k_err_t err;
    mail_t mail;
    size_t mail_size;

    while (K_TRUE) &#123;
        err = tos_mail_q_pend(&amp;mail_q, &amp;mail, &amp;mail_size, TOS_TIME_FOREVER);
        if (err == K_ERR_NONE) &#123;
            TOS_ASSERT(mail_size == sizeof(mail_t));
            printf(&quot;higher: msg incoming[%s], payload[%d]\n&quot;, mail.message, mail.payload);
        &#125;
    &#125;
&#125;

void entry_task_receiver_lower_prio(void *arg)
&#123;
    k_err_t err;
    mail_t mail;
    size_t mail_size;

    while (K_TRUE) &#123;
        err = tos_mail_q_pend(&amp;mail_q, &amp;mail, &amp;mail_size, TOS_TIME_FOREVER);
        if (err == K_ERR_NONE) &#123;
            TOS_ASSERT(mail_size == sizeof(mail_t));
            printf(&quot;lower: msg incoming[%s], payload[%d]\n&quot;, mail.message, mail.payload);
        &#125;
    &#125;
&#125;

void entry_task_sender(void *arg)
&#123;
    int i = 1;
    mail_t mail;

    while (K_TRUE) &#123;
        if (i == 2) &#123;
            printf(&quot;sender: send a mail to one receiver, and shoud be the highest priority one\n&quot;);
            mail.message = &quot;1st time post&quot;;
            mail.payload = 1;
            tos_mail_q_post(&amp;mail_q, &amp;mail, sizeof(mail_t));
        &#125;
        if (i == 3) &#123;
            printf(&quot;sender: send a message to all recevier\n&quot;);
            mail.message = &quot;2nd time post&quot;;
            mail.payload = 2;
            tos_mail_q_post_all(&amp;mail_q, &amp;mail, sizeof(mail_t));
        &#125;
        if (i == 4) &#123;
            printf(&quot;sender: send a message to one receiver, and shoud be the highest priority one\n&quot;);
            mail.message = &quot;3rd time post&quot;;
            mail.payload = 3;
            tos_mail_q_post(&amp;mail_q, &amp;mail, sizeof(mail_t));
        &#125;
        if (i == 5) &#123;
            printf(&quot;sender: send a message to all recevier\n&quot;);
            mail.message = &quot;4th time post&quot;;
            mail.payload = 4;
            tos_mail_q_post_all(&amp;mail_q, &amp;mail, sizeof(mail_t));
        &#125;
        tos_task_delay(1000);
        ++i;
    &#125;
&#125;
</code></pre>
<p>运行结果为：</p>
<pre><code class="c">TencentOS-tiny Port on STM32L431RCT6 By Mculover666

sender: send a mail to one receiver, and shoud be the highest priority one
higher: msg incoming[1st time post], payload[1]

sender: send a message to all recevier
higher: msg incoming[2nd time post], payload[2]
lower: msg incoming[2nd time post], payload[2]

sender: send a message to one receiver, and shoud be the highest priority one
higher: msg incoming[3rd time post], payload[3]

sender: send a message to all recevier
higher: msg incoming[4th time post], payload[4]
lower: msg incoming[4th time post], payload[4]
</code></pre>
<blockquote>
<p>此示例主要演示了两点：1. 如何使用邮箱队列直接传递值；2. 唤醒一个等待任务和唤醒所有等待任务的区别。</p>
</blockquote>
<h1 id="5-优先级邮箱队列"><a href="#5-优先级邮箱队列" class="headerlink" title="5. 优先级邮箱队列"></a>5. 优先级邮箱队列</h1><p>看到这里，这个不能再讲了吧~</p>
<p>TencentOS-tiny中实现在<code>tos_priority_mail_queue.c</code> 和<code>tos_priority_mail_queue.h</code>中。</p>
<p>可以自己尝试根据前面的demo，编写出一个使用优先级邮箱队列的demo，测试高优先级的邮件是否会被先收到，然后将结果与第4节的实验结果进行对比。</p>
<p>越到文末我越浪，划水已经不能满足了，博主要去摸鱼~</p>
<h1 id="6-总结"><a href="#6-总结" class="headerlink" title="6. 总结"></a>6. 总结</h1><p>按照惯例，对本文所讲的内容进行一个总结。</p>
<p>本文主要讲述了用于任务间通信的一些内核对象，主要有四个：消息队列和优先级消息队列，邮箱队列和优先级邮箱队列。</p>
<p>接下来列出一些重要的点：</p>
<p>① <strong>在使用RTOS中的一些用于任务间通信的量时，要注意传递的是值还是地址。TencentOS-tiny中消息队列传输的是地址，而邮箱队列传递的是值。</strong></p>
<p>② <strong>消息队列和邮箱队列基于环形队列实现，遵循FIFO规则；而优先级消息队列和优先级邮箱队列基于优先级队列实现，遵循按照元素优先级取出的规则。</strong></p>
<p>最后来回答题目中的问题：任务间通信为什么不使用全局变量？</p>
<p>① 无论是消息队列还是邮箱队列，都是利用了全局变量可以被随意访问的特性，所以使用时都会被定义为全局变量。</p>
<p>② 普通全局变量可用于一些简单的任务间通信场合。</p>
<p>③ 相较于普通全局变量，加入队列机制可以存储多个消息，加入pend-post机制可以拥有任务等待和唤醒的机制，用于解决队列已满或队列为空的问题。</p>
<p>本文就讲到这里啦~我是喜欢玩板子的小码农Mculover666，下期文章再见！</p>

                
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